Habitat and dietary selectivity of nearshore shark populations

Abstract

The resource use strategies species exhibit affects their role within communities and how they respond to environmental change. Species that adopt generalist strategies are typically less vulnerable to environmental fluctuations than specialists. However, specialists often have lower energy costs and are more efficient at extracting and processing preferred resources. This dissertation defines shark ecological specialisation in order to evaluate the resource use patterns of nearshore sharks and discusses how shark resource use patterns can affect their vulnerability to environmental change.

Sharks are traditionally classified as generalists that use a variety of habitats and prey. While this is an accurate description of some species, sharks exhibit a range of resource use strategies that includes highly selective or specialised behaviours. However, discussion on how to define the ecological specialisation of sharks has been limited. This dissertation presents a conceptual framework within which to define the specialisation of sharks that can be applied to different environmental scales. Shark species with varying degrees of specialisation are presented within the proposed context.

Passive acoustic telemetry was used to examine the residency, space use, and habitat selection and specialisation patterns of the small-bodied Australian sharpnose shark, Rhizoprionodon taylori, and the medium-bodied creek whaler Carcharhinus fitzroyensis, in Cleveland Bay, Queensland, Australia. Stable isotope analysis of δ¹³C (¹³C/¹²C) and δ¹⁵N (¹⁵N/¹⁴N) was used to define the regional nearshore residency, movements, trophic level, and benthic and pelagic contributions to the diet of R. taylori. δ¹³C values vary at the base of the food chain but are conserved up the food chain. δ¹⁵N values increase in predictable quantities between trophic levels. Therefore, δ¹³C and δ¹⁵N values can be used to indicate the foraging location and diet of populations. Plasma and muscle δ¹³C and δ¹⁵N of R. taylori were collected from five embayments, including Cleveland Bay, on the northeast coast of Queensland.

Results of the acoustic tracking showed the majority of R. taylori were present in Cleveland Bay for short periods of time, ranging from 1 to 112 days (mean ± SE = 17 ± 5). The majority of individuals were present in Cleveland Bay for less than two weeks. Low residency suggests that R. taylori home ranges likely span multiple bays. Individual monthly activity space ranged from 4.3 and 21.4 km² (mean ± SE =11.3 km² ± 0.90) for 50% kernel utilisation distributions (KUDs) and 21.5 and 80.4 km² (mean ± SE = 51.0 km² ± 3.9) for 95% KUDs. Space use analysis indicated R. taylori roamed widely throughout the bay, but monthly activity space size was consistent among individuals and over time. Sex and size had no influence on R. taylori residency or activity space size. Both the population and individuals occupied wide habitat niches which included seagrass, outer bay mud substrate, and sandy inshore habitat. However, both resident and transitory R. taylori consistently selected for seagrass over other habitats, potentially for feeding. Mudflat and reef habitats were generally avoided. Habitat selection appeared to be influenced by changes in freshwater input into Cleveland Bay. Selection for seagrass habitat, which is adjacent to large river mouths, decreased during periods of high river flow, suggesting R. taylori may have limited tolerance to low salinity.

Stable isotope analysis showed that there was a positive correlation between R. taylori tissue and environmental (seagrass and plankton) δ¹³C values based on location. Shark populations with the highest tissue δ¹⁵N were collected from areas with the highest baseline δ¹⁵N values. Moreover, populations from bays that were > 100 km apart had distinct isotopic values. These results indicate R. taylori were not foraging more than 100 km from their capture location within 6 to 12 months. However, δ¹³C values of individuals in nearby bays (30-70 km apart) were indistinguishable, suggesting individuals foraged and moved between bays that were within 100 km of each other during a 6 to 12 month period. Therefore, isotope results were consistent with the low residency exhibited by R. taylori in Cleveland Bay. Isotope analysis also revealed R. taylori had a wide trophic range and consumed prey from benthic and pelagic sources. In all areas, benthic sources were important to the diet, suggesting benthic habitats (e.g. seagrass) may be important to R. taylori. However, there was geographic and temporal variation in R. taylori diet. These results indicate R. taylori has a broad dietary niche, but different populations may have unique effects on distinct areas. Variation in diet also suggests R. taylori may be adaptive to changes in prey availability.

In contrast to R. taylori, most C. fitzroyensis were highly resident and present in Cleveland Bay for long periods of time, ranging 1 to 452 days (mean ± SE = 205 ± 53). However, a few individuals spent less than two weeks in the bay, suggesting broader movements occur in a portion of the population. Size and sex had no effect on presence. Individual monthly activity space ranged from 2.6 to 19.8 km² (mean ± SE =10.6 km² ± 0.3) for 50% KUDs and 9.1 to 81.9 km² (mean ± SE = 47.9 km² ± 1.0) for 95% KUDs. Activity space size varied between months and diel period but was not affected by animal size. Activity spaces in August were significantly smaller and concentrated closer to the shore than in other months. This simultaneous shift in space use by all individuals may have been to due biotic changes in the bay, such as changes in prey availability. Larger day time activity spaces suggest C. fitzroyensis may be primarily diurnal feeders. All resident C. fitzroyensis spent the majority of time in seagrass and to a lesser extent outer bay mud substrate habitat. Seagrass was consistently selected for throughout the monitoring period while use of outer bay mud substrate was highly irregular. Shallow mudflat, sandy inshore, and reef habitats were rarely used. There was no difference in space or habitat use between immature and mature individuals, indicating different age classes shared space and habitats.

The results of acoustic and isotope analyses indicated that seagrass habitat is the preferred habitat of R. taylori and C. fitzroyensis. Seagrass is typically highly productive and may be an important foraging habitat for these species. For that reason, seagrass conservation will be an important consideration for the future spatial management of these species. However, R. taylori used different habitats and embayments and had a broad diet. Therefore, results indicate R. taylori has a low degree of resource specialisation and is probably adaptive to local environmental change. In contrast, the movement patterns exhibited by C. fitzroyensis suggest this species has a moderately high degree of habitat specialisation and is highly resident. Although diet information is not available for C. fitzroyensis, the movement patterns of C. fitzroyensis indicate this species will likely be more vulnerable to local environmental change, specifically a decline in seagrass abundance. By evaluating the resource use patterns of nearshore sharks, this dissertation has provided valuable information on the potential vulnerabilities of poorly understood shark species while also developing a conceptual framework for future resource specialisation investigations.